The present invention relates to a rolling bearing. More particularly, the present invention relates to a grease-lubricated bearing for engine auxiliary machinery (e.g., alternator, solenoid clutch, inter-pulley, compressor for car air conditioner, water pump).
In recent years, as the size and weight of automobiles have decreased, engine auxiliary machinery has been called for lower size and weight as well as higher performance and output. For example, a bearing for alternator is acted upon by high vibration and high load (acceleration of gravity: approx. 4 G to 20 G) accompanying a high speed rotation at the same time with the actuation of the engine through a belt. Accordingly, a grease-lubricated bearing, if used for alternator, is liable to seizing that causes locking. Such a grease-lubricated bearing for alternator is also disadvantageous in that the bearing surface (so called as a rolling contact surface which is brought in contact with rolling elements) of the outer ring as a stationary race is liable to premature peeling. As a result, such a grease-lubricated bearing for alternator cannot be provided with a sufficiently prolonged life.
As a technique for prolonging the life of bearings operating under high vibration and high load there is disclosed a process which comprises subjecting a stationary race made of a high carbon-chrome bearing steel (SUJ2) to normal hardening, subjecting the stationary race thus hardened to sub-zero treatment, and then subjecting the stationary race to high temperature tempering to reduce the content of residual austenite in the stationary race to not more than 10 vol-% in a Japanese Patent Examined Publication Hei. 7-72565 (hereinafter referring to JP-B-7-72565). In other words, this process is intended to reduce the content of residual austenite in the stationary race so that the hardness of the stationary race is kept high and the plastic deformation of the bearing surface of the stationary race under high vibration and high load is minimized, thereby preventing premature peeling.
As a countermeasure against premature peeling of stationary race there is disclosed in Paragraphs 1 to 14 of "SAE Technical Paper: SAE950944 (held on Feb. 27-Mar. 2, 1995) an explicated mechanism of fatigue of bearing for alternator on the basis of which the grease to be enclosed is changed from E grease to M grease. Because of its high damping effect, M grease can sufficiently absorb vibration and load to prevent premature peeling of the stationary race when applied to bearings operating under high vibration and high load.
On the other hand, a Japanese Patent Examined Publication Hei. 6-33441 (hereinafter referring to JP-B-6-33441) discloses a technique which comprises forming a bearing ring by a steel containing C in an amount of from 0.95 to 1.10% by weight, Si or Al in an amount of from 1 to 2% by weight, Mn in an amount of not more than 0.50% by weight and Cr in an amount of from 0.90 to 1.60% by weight and having an oxygen content of not more than 13 ppm, hardening the bearing ring, and then tempering the bearing ring at a temperature as high as 230.degree. C. to 300.degree. C. to adjust the content of residual austenite and the surface hardness HRC thereof to not more than 8 vol-% and not less than 60, respectively. An object of this technique is to obtain a bearing having a high dimensional stability and a prolonged rolling life in operation at high temperatures.
Further, a Japanese Patent Unexamined Publication No. Hei. 7-103241 (hereinafter referring to JP-A-7-103241) discloses a technique involving the adjustment of the content of residual austenite in the bearing ring of a bearing made of a bearing steel or stainless steel to not more than 6 vol-%. This technique applies to rolling bearing for HDD or audio equipment and is intended to prevent the occurrence of impression on the bearing surface that causes vibration during use, making it possible to improve the acoustic characteristics of the equipment.
However, the techniques disclosed in the JP-B-7-72565 and SAE Technical Paper can exert an effect of preventing premature peeling of the stationary race of a bearing operating under high vibration and high load but leaves something to be desired in the resistance of bearing for engine auxiliary machinery to seizing during use at high temperatures.
In other words, when such a bearing is used at an ambient temperature as high as not lower than 100.degree. C. over an extended period of time, the inner ring as a rotating race tends to have a temperature of 10.degree. C. or more higher than that of the outer ring as a stationary race. This is attributed to the fact that the frictional heat developed on the bearing during rotation can be dissipated more fairly through the housing, to which the outer ring is fixed, than through the shaft, to which the inner ring is fixed. Thus, the decomposed amount of residual austenite is greater in the inner ring, which has a higher temperature than the outer ring, than in the outer ring. This causes an increase in the bearing diameter of the inner ring, causing a decrease in the gap of the bearing. As a result, seizing can easily occur.
Further, the JP-B-6-33441 and the JP-A-7-103241 have no description intended for the prevention of the foregoing seizing during use at high temperatures.
Moreover, in the technique described in the JP-B-6-33441, the oxygen content is defined to not more than 13 ppm because the content of Si or Al in the steel constituting the stationary race is as much as 1 to 2% by weight. However, this technique is liable to production of huge silicon-based or alumina-based inclusions causing remarkable deterioration of rolling life.
Further, the technique described in the JP-A-7-103241 can effectively apply to ball bearing having a pitch circle having a diameter of not more than 11 mm in which a plurality of balls having a diameter of not more than 3 mm are arranged such as small-sized ball bearing and miniature ball bearing having an inner diameter of less than 10 mm but leaves something to be desired in peeling resistance and seizing resistance when applied to bearing for engine auxiliary machinery having an inner diameter of not less than 10 mm which operates at higher temperatures under higher vibration than for HDD or audio equipment.